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Article source 'Ian Richards, FACTS Technical Information. For details of FACTS membership, telephone 01335 343945'

SOIL ACIDITY AND LIMING

Crop problems due to soil acidity and the value of liming have been recognised, if not understood, for centuries. Today, there is a reasonable understanding of the chemistry underlying soil acidity and of the ways in which acidity harms crops. The usual problems related to severe acidity (pH < 5) are the toxicity to plants of the aluminium and manganese made available in these conditions. The availability of phosphorus also is reduced at low soil pH due to the formation of insoluble iron and aluminium phosphates. Other problems, such as molybdenum deficiency or club-root in brassicas, can occur in acid soils.

Soil Acidity

Soil pH is a measure of the concentration of hydrogen ions (H⁺) in solution. This concentration is measured in pH tests and pH is defined as the negative of the logarithm of the hydrogen ion concentration:

pH = -log (H⁺ concentration)

This gives a convenient scale for pH of 0 to 14 with 7 equivalent to neutrality (neither acid nor alkaline). Values less than 7 indicate acidity and those greater than 7 indicate alkalinity.

There are two components to total soil acidity:

• Active acidity due to hydrogen ions in solution in the soil.
• Reserve or potential acidity due to hydrogen and aluminium ions attached to cation exchange sites in the soil. These ions can become displaced from exchange sites into solution and so add to active acidity. Soils with high cation exchange capacity (CEC) can have large reserve acidity if unlimed.

Reserve acidity can buffer changes in active acidity so, to change soil pH by a given amount, lime must be applied to neutralise both active and reserve acidity. This is why soils with large CEC, for example soils with a high clay or organic matter content, require larger amounts of lime to raise pH by a particular number of units (they have a large soil pH buffer capacity).

Active acidity is measured in pH tests where soil is mixed with water (the usual method in England, Wales and Northern Ireland). Measurement of lime requirement (tonnes/ha of calcium carbonate needed to raise soil pH to a target value) must take into account both pH and the pH buffer capacity of the soil. This can be done approximately using soil pH and texture (the usual method now in England and Wales) or it can be measured in the laboratory. The laboratory measurement involves mixing the soil with a buffer solution of high pH. If the soil has a small buffer capacity, the pH of the mixture will remain close to that of the buffer solution. If the soil has a large buffer capacity, the pH of the mixture will be less than that of the buffer solution. Lime requirement is calculated from the pH of the mixture.

In Scotland, soil pH often is measured in a suspension of soil in calcium chloride solution rather than in soil/water. The calcium chloride displaces some of the hydrogen and aluminium ions from exchange sites so this method measures active plus a proportion of reserve acidity. The measured pH is lower than that measured in soil/water, usually by around 0.6 pH units. However, analytical reports include a correction so that the pH result is equivalent to that measured in soil/water.

Acidification of Soils

Natural processes tend to acidify most soils in the UK:

• Decomposition of soil organic matter that releases carbon dioxide. This dissolves in soil water to form carbonic acid.
• Mineralisation of nitrogen in organic matter or nitrification of ammonium-N applied in, or derived from, fertilizers. The oxidation of ammonium-N to nitrate-N releases hydrogen ions (NH₄⁺ + 2O₂ NO₃- + H₂O + 2H⁺). If the nitrate is taken up by plants, the acidity can be reduced by hydroxyl (OH-) ions exuded from roots (these combine with hydrogen ions to form water). Net acidification of the soil occurs where the nitrate is lost by leaching rather than by plant uptake.
• Release of hydrogen ions from roots to maintain cation/anion balance within the plant as cations are taken up by roots.
• Addition of carbonic, nitric and sulphuric acids in low concentrations in rain (pH of rainwater is around 5.5).
• Crop removal of basic cations (potassium, calcium, magnesium and sodium).
• Leaching of basic cations.

Reducing Acidity

An effective liming material must do two things: it must displace hydrogen and aluminium ions from exchange sites and it must then neutralise the resulting hydrogen ions when they are in soil solution. The calcium in calcium carbonate (limestone, chalk) displaces hydrogen and aluminium ions from exchange sites. The magnesium in magnesium carbonate (dolomitic or magnesian limestone) has the same function. The carbonate in either of these materials then neutralises the hydrogen ions.

In more detail, when added to soil, calcium carbonate reacts with hydrogen ions (H⁺) in solution to form carbonic acid (H₂CO₃) and calcium ions. The carbonic acid can convert to carbon dioxide, which is lost to the air, and to water:

CaCO₃ + 2H⁺ H₂CO₃ + Ca²⁺ CO₂ + H₂O + Ca²⁺

The calcium ions that have been released into solution can displace hydrogen and aluminium ions from exchange sites (one calcium ion displaces two hydrogen ions, three calcium ions displace two aluminium ions). When aluminium ions (Al³⁺) are displaced from exchange sites, they enter a series of reactions with water that generate hydrogen ions:

Al³⁺ + H₂O Al(OH)²⁺ + H⁺
Al(OH)²⁺ + H₂O Al(OH)₂⁺ + H⁺
Al(OH)₂⁺ + H₂O Al(OH)₃ + H⁺

The hydrogen ions then react with calcium carbonate and are neutralized. The overall effect of adding calcium carbonate to an acidic soil is a reduction in the concentration of hydrogen ions (both in solution and on exchange sites) and so an increase in soil pH. Magnesium carbonate reacts in the same way, magnesium replacing calcium in the reactions.

Effectiveness and Quality of Liming Materials

In the UK, the effectiveness of liming materials is described by neutralising value and particle size. Generally, the higher the neutralising value and the smaller the particle size, the more effective is the liming material. However, hardness also is important and small particles of limestone can be less effective than larger particles of softer chalk. Measures of reactivity of liming materials are used in several European countries but not widely, as yet, in the UK. However, a British Standard for a reactivity test has been published.

The quality of liming materials remains covered by The Fertilisers Regulations 1991, as amended. These materials are not covered by EC Regulation 2003/2003 that has superseded The Fertilisers Regulations for most fertilizers. For all liming materials, both the neutralising value and the specific material name must be declared. In addition, for the various grades of limestone and magnesian (dolomitic) limestone and furnace slag, the percentage by weight passing through a 150 micron sieve must be declared. The percentage by weight of material passing through a 6.3 mm sieve must be declared for mixed lime, and the percentages passing through 5 mm, 3.35 mm and 150 micron sieves must be declared for any liming materials not specifically listed in The Fertilisers Regulations. The limits of variation are 5% of the amount stated.

Liming Materials

In the UK, liming materials are generally treated as bulk commodities, supplied as crushed limestone or chalk. Soil pH monitoring should be a regular part of nutrient management and lime should be applied according to soil and rotational requirements. Typically, lime is applied at intervals of three years or more at rates of around 3 to 5 t/ha. In some areas, application of magnesian limestone over many years has raised soil magnesium to high indices. Where this has occurred, a change to calcitic limestone or chalk should be made if possible. Granular lime products, capable of being spread by farm equipment, are available in the UK and are establishing a role alongside conventional materials. These products consist of very fine lime particles (< 150 micron) pressed with a binder into 2 – 5mm granules. Owing to the small particle size, granular products are quick-acting but the lower amounts applied can not be expected to achieve the same durability as conventional liming in displacing hydrogen and aluminium ions. Consequently, where soil pH needs to be raised from, say, 5.5 to 6.5 then an application of limestone or chalk will be most appropriate. However, granular products can be applied annually or every other year by farm spreaders to maintain pH. These products also are particularly suitable for treating patches of acidity within fields.

Increasing Acidity

It is not practical to increase acidity of calcareous soils and the familiar problems of micronutrient deficiencies in crops grown on these soils must be managed in other ways. Non-calcareous soils tend to acidify naturally and this process can be accelerated by using ammonium-N fertilizers. In some circumstances, pH can be reduced in localised areas within the soil by application of an acidifying material. Elemental sulphur can have this effect as it oxidises and releases hydrogen ions:

2S + 3O₂ + 2H₂O 4H+ + 2SO₄^2-

Further Information

Information on liming and liming materials is at the web site of the Agricultural Lime Association, www.aglime.org.uk.

A good review of lime and liming is in Goulding K W T and Annis B (1998) Lime, liming and the management of soil acidity, Proceedings No. 410, The International Fertiliser Society, York. Copies of this (£12.50 to non-members of the Society) are available from:

The International Fertiliser Society
PO Box 4
York
YO32 5YS

Email secretary@fertiliser-society.org

The Rothlime model for calculating lime requirement under UK conditions can be downloaded from the Rothamsted Research web site: www.rothamsted.bbsrc.ac.uk/aen/rothlime/.

Rothlime takes into account the effects of acid rain so gives liming recommendations that are slightly higher than those in RB209. Versions of Rothlime also are on the EMA CD and on CD from the Agricultural Lime Association.

Agricultural Lime Association C/O Agricultural Lime Federation Confederation House East of England Showground Peterborough PE2 6XE

Tel: 01733 385240 Fax: 01733 385270 Email: eileen.pullinger@agindustries.org.uk